The objective of the proposed research is to determine the molecular mechanism of ionizing radiation-induced mitotic delay. Although previous research has focused on the relation between protein synthesis and mitotic delay, we will test alternative hypotheses concerning the control of phosphorylation of division-related proteins (histone H1 and mitotic spindle proteins). The following cell cycle markers will be precisely timed in late G2-prophase: physiological markers (the time when the cell loses sensitivity to cycloheximide (CH), to ionizing radiation, and to radiation and CH in combination); cytological markers (including chromosome condensation, dissolution of the nucleolus, and assembly of the bipolar mitotic spindle); and biochemical markers (variations in cyclic nucleotide content, histone phosphorylation (primarily H1), and phosphorylation of proteins of the mitotic spindle). Attempts will be made to relate each physiological marker to a cytological and/or biochemical marker by comparing: a) their times of occurrence in the normal cell cycle, b) sensitivities to CH and/or radiation, c) radiation and drug dose-responses. d) reversals or modifications by specific agents. The acellular slime mold Physarum polycephalum provides an ideal system for this study, since nuclear divisions are much more highly synchronous than in most other cell types, including mammalian cells, and large amounts of synchronous material necessary for biochemical analysis can be readily obtained. The information obtained in the proposed experiments should help to elucidate the factors which control radiation-induced mitotic delay, and the specific action of some drugs which modify it. Such knowledge might provide a conceptual framework for understanding diseases such as cancer which are characterized by uncontrolled cell division.